The fabrication of semiconductor devices often requires the careful synchronization and precisely measured delivery of as many as a dozen gases to a processing tool and/or chamber. Various recipes are used in the fabrication process, and many discrete processing steps, where for example a semiconductor device is cleaned, polished, oxidized, masked, etched, doped, metalized, can be required. The steps used, their particular sequence and the materials involved all contribute to the making of particular devices.
Accordingly, wafer fabrication facilities are commonly organized to include areas in which chemical vapor deposition, plasma deposition, plasma etching, sputtering and other similar gas manufacturing processes are carried out. The processing tools, be they chemical vapor deposition reactors, vacuum sputtering machines, plasma etchers or plasma enhanced chemical vapor deposition chambers, etc. must be supplied with various process gases. Pure gases must be supplied to the tools in contaminant-free, precisely metered quantities.
In a typical wafer fabrication facility the gases are stored in tanks, which are connected via piping or conduit to a gas delivery system. The gas delivery system includes a gas box for delivering contaminant-free, precisely metered quantities of pure inert or reactant gases from the tanks of the fabrication facility to a process tool and/or chamber. The gas box typically includes a plurality of gas flow lines each having a gas metering unit, which in turn can include valves, pressure regulators and transducers, mass flow controllers and filters/purifiers. Each gas line has its own inlet for connection to separate sources of gas, but all of the gas paths converge into a single outlet for connection to the process tool.
Sometimes dividing or splitting the combined process gases among multiple processing tools and/or chambers is desired. In such cases, the single outlet of the gas box is connected to multiple process tools and/or chambers through secondary flow lines. In some applications, where for example, the upstream pressure needs to be kept lower (e.g., 15 PSIA) for safety or other reasons, a flow ratio controller is used to insure that the primary flow of the outlet of the gas box is divided in accordance with a preselected ratio among the secondary flow paths. Examples of split flow systems are described in U.S. Pat. Nos. 4,369,031; 5,453,124; 6,333,272; 6,418,954 and 6,766,260; and published U.S. Application No. 2002/0038669. The flow ratio controller of U.S. Pat. No. 6,766,260 is of particular interest because each second flow line is controlled with a separate flow meter and control valve.
Flow ratio controllers of the type shown in U.S. Pat. No. 6,766,260 will quickly stabilize to the desirable ratio split when initially set, but flows take time to be stabilized, and in some applications this can be unsatisfactory. Further, the pressure drop across the flow ratio controller is high, and the controller provides poor control performance for handling downstream blocking of one of the secondary flow paths. Additionally, the system can be difficult to set up because of difficulties in initially determining fixed valve positions of the valves in the secondary flow lines. And for embodiments using two secondary flow lines it is necessary to assign the high flow valve as the fixed valve and the low flow valve as the controlled valve for flow ratio control.